since jacob rees mogg has decided that petitions aren't valid, only petitions hosted on the official gov petition site are, here's a masterlist of all blm/blm adjacent petitions that i can find.
you do need a uk postal code to sign these petitions, so if you're outside of the uk then you could find one online, although i don't know how much i'd recommend that, since the government's already being finnicky about what petitions are valid, and i don't want to give them a reason to find these petitions invalid.
from pinkyytheelf's thread:

Teach Britain's colonial past as part of the UK's compulsory curriculum
Improve Maternal Mortality Rates and Health Care for Black Women in the U.K.
Add education on diversity and racism to all school curriculums
Introduce Mandatory Ethnicity Pay Gap Reporting
Condemn the US government for the use of force against its citizens

from the comments on that thread:

Create an independent investigatory commission to help protect ethnic minorities
Making the UK eduughion curriculum more inclusive of BAME history
(Wales) Make it compulsory for Black and POC UK histories to be taught in the Welsh education curriculum.
Shukri Abdi (This one isn't approved yet)